Conventionally, when electronic components which emit heat such as transistors and diodes were installed, thermally conductive grease was applied between heat sinks and the metal chassis in order to confer insulating or thermally conducting properties. The application of this grease however required a complicated procedure, and as it is also tended to cause soiling, electrical insulating sheets with good thermally conductive properties subsequently came into use.
With the recent generalization of insulating type full pack mold transistors wherein the whole of the electronic device is sealed in by an epoxy resin, thermally conductive sheets have now come to replace electrical insulating sheets with thermally conductive properties. This is due to the fact that, as electronic components which emit heat are now full pack molded, insulating properties are no longer necessary and thermally conductive properties are an important factor to be considered.
Conventional thermally conductive sheets use rubber or plastic as binders with metal powders or inorganic powders with good thermally conductive properties as fillers.
Examples of inorganic powders with good thermally conductive properties are alumina (thermal conductivity: 0.074 cal/cm.sec. .degree.C.), boron nitride (thermal conductivity: 0.15 cal/cm.sec. .degree.C.), magnesium oxide (thermal conductivity: 0.12 cal/cm.sec. .degree.C.), and silicon carbide (thermal conductivity: 0.22 cal/cm.sec. .degree.C.), but these materials suffered from the disadvantage of having poorer thermal conductivity than that of metal powders.
Examples of metal powders are aluminum (thermal conductivity: 0.57 cal/cm.sec. .degree.C.), copper (thermal conductivity: 0.95 cal/cm.sec. .degree.C.) and silver (thermal conductivity: 1.02 cal/cm.sec. .degree.C.). Aluminum and copper are however easily oxidized, and although silver is not an easily oxidizable metal, it suffers from the disadvantage of very high cost.
In recent years, a high thermally conductive sheet using a powder or short strands of aluminum or copper metal (Japanese Patent Laid-Open (KOKAI) No. 58-163623) and a thermally conductive sheet consisting of a graphite sheet coated on either one or both sides with a silicone rubber (Japanese Patent Laid-Open (KOKAI) No. 62-25440) have been proposed. In the former case, however, thermal conductivity decreased with time due to oxidation of the metal powder and the strength of the sheet was unsatisfactory. In the latter case, thermal conductivity was high and did not decrease with time, however the graphite sheet was costly and the cost of manufacturing it was high.
Further, conventional thermally conductive sheets have no adhesive properties, so when transistors were fitted to heat sinks or other components, a positional maladjustment easily occurred between the thermally conductive sheet and the transistor. The correct setting of components was thus an extremely troublesome operation.
The inventors of the present invention carried out extensive studies to resolve the aforesaid disadvantages.
As a result, it has been found that by providing a silicone rubber layer containing graphite powder (thermal conductivity 0.31 cal/cm.sec. .degree.C.) on one or both sides of a metal foil, a low cost thermally conductive sheet can be obtained having far superior thermal conductivity compared to conventional thermally conductive sheets using inorganic powders, and that this thermally conductive sheet can prevent a decrease of thermal conductivity with time due to oxidation as occurred with thermally conductive sheets using metal powders. Also, it has been found that by adding an adhesion-conferring organopolysiloxane to the silicone rubber in order to improve the contact between the transistor and the heat sink, the stability of the assembly can be improved, and a thermally conductive sheet with improved thermal conductive properties can be obtained even when there are scratches or marks on the transistor.